The above identified parent application describes welding electrodes with a round weld face for contact with a metal workpiece in an electrical resistance welding operation. The weld face includes concentric rings of ridges and/or grooves extending radially from the center of the weld face; the ridges extending axially upwardly from the face and the grooves extending inwardly into the face. When the face of the electrode is pressed into contact with a surface of the metal workpiece for delivery of a welding current, these shaped features on the face of the electrode penetrated surface oxides or other conductivity barriers to facilitate passage of a welding current.
Current automotive vehicle manufacturing operations include, for example, the joining of two sheet metal layers by spot welding. Vehicle body panels such as doors, hoods, deck lids and liftgates are often assembled by joining inner and outer panels stamped from sheet metal of suitable metal alloys. Ferrous or aluminum alloys are often used. The thickness of each sheet metal layer may vary from less than one millimeter to more than four millimeters. Electrical resistance spot welding is often used to join such inner and outer panels or other metal parts. For example, an edge of an outer panel sheet may be folded over a complementary edge of an inner panel sheet in an assembly of the panels in which the hem(s) is at the periphery of the sheets. The panel assembly is positioned for welding in areas removed from the hem joint. Axially aligned and opposing electrodes are pressed toward each other against opposite sides of the panel assembly. A momentary welding current is passed between the electrodes through the layers of metal to form a spot weld. The spot weld is characterized by a momentarily fused pool of metal and a re-solidified weld nugget at the interface of the contacting sheets. The electrodes are retracted and moved to another weld site. The welding electrodes of the parent application with concentric features on their welding faces form high quality welds in metal workpieces.
In some applications spot weld sites may be visible in the surface of a finished product such as an automotive vehicle body panel or member. Several types of joints are used to join sheet metal panels in areas that may be visible to customers. These areas include joints formed on vehicle body closure members such as hoods, decklids, liftgates, and doors. Visible joints in the vehicle structure also include those in the engine compartment and trunk. It has been observed that the welding electrodes of the parent application not only form strong durable spot welds in such welded articles, but the circular ridges or grooves leave a faithful impression on a visible surface of a spot weld site. The clear visible ring pattern in the surface of the welded assembly is found to be much more pleasing to a consumer than the traditionally formed welding electrode impression. Such a pleasing weld appearance can also influence the perceived quality of the assembly, that is, the appearance details alone can influence the consumer's perception as to whether a product will fulfill his or her expectations. And where a progressive sequence of such weld sites is visible, the suggestion of high quality is reinforced.
Resistance spot weld appearance can suffer from several undesirable features when using current, conventional spot weld electrodes. For example, a domed electrode with a flat welding face machined in its center is widely used for spot welding steel. It tends to leave a sharp imprint on the sheet surface that can have excessive indentation especially if the electrode engages the sheet surface in an off-normal orientation when forming a spot weld. The electrode shape is also prone to causing sheet deformation around the weld that is unattractive. Metal expulsion from excessive heat and off-normal orientation can lead to whiskers or fingers of metal protruding from the sheet surface which is undesirable.
Such welding surface issues have not arisen when using the electrodes of the parent application. So it is an object of this invention to provide additional welding electrode face features that assure the formation of high strength or structural spot welds in workpieces, such as steel and aluminum sheet metal workpieces, while leaving behind clear visible indicia in a weld surface that provides an attractive appearance.